Dynamic Demand Control (DDC) is conventionally known as a demand side management technique where the frequency of the utility supply, i.e. the grid frequency, is allowed to vary over a small range in response to fluctuations in the power being generated compared with the power being used at that moment. If the available power is too high the grid frequency is allowed to increase by a small amount; if the available power is too small the grid frequency is allowed to reduce. The grid may be viewed as a huge spinning load and these changes in frequency correspond to changes in the rotational speed of that load and are large energy fluctuations. If the frequency is too high then non-essential load can be switched on to absorb some of that energy; if it is too low then non-essential load can be switched off to free up spinning power for more important applications. Unless the context clearly requires otherwise, references to “DCC” herein refer to such a system.
In a practical situation a large number of small DDC capable loads, each with its own controller, are distributed over the network. As the network frequency varies each controller determines what load is called for and switches that fractional part load on. As shown in FIG. 1 if the frequency is less than 49 Hz the load switched is zero (W), if it is greater than 51 Hz the load is the full rated load, and between these two extremes the load varies linearly. Thus if there were 1 million of these devices on the network the actual load applied would be a resistive load variable from 0 to 2 GW. It should be noted that this is an example only and in practice the range 49-51 Hz would be a lot smaller, and not all the loads have to be the same. The prime requirement is that the DDC capable loads can be switched in a continuously variable way between 49 and 51 Hz—or at least switched on and off inside that range.
There are limitations on the type of load that can be made DDC compliant. In general ‘energy’ loads such as water heaters, battery chargers, freezers, refrigerators, and air conditioners are suitable but care must be taken where such loads include motors, pumps, and fans as rapid switching of these devices on and off may adversely affect their life. Nonetheless DDC compliant energy loads make up a significant fraction of the electric load on any grid system and make DDC an attractive technology to implement.
DDC is implemented in the simplest possible way by allowing the mains frequency to vary in response to loads. Schematically the whole grid can be replaced with a generator with inertia J and a load that varies with frequency shown in FIG. 2. A prime mover with no other controller drives the inertia J representing the Grid and DDC compliant loads (not shown) connected to the generator moderate the net torque driving the inertia J, via the feedback path.
SymbolDefinitionΔτChange in input torqueJSystem inertiaΔωSystem frequencykConstant for conversion between frequency and torqueTFilter time constant
Here changes in the input torque driving the system cause the system to change speed (frequency) according to the system inertia J. Changes in the speed are observed and used to change the load on the network to control the change in speed. Practically, there must be at least some filtering on the frequency measurement to remove non-linear effects caused by armature reactance changes and other disturbances. In fact without this filtering control is impossible. Thus aDDC controller has a feedback signal of
  k      1    +    sT  instead of simply k. In a practical application where generation is at 50 or 60 Hz this filtering may be achieved with a narrow band single pole band pass filter on the AC waveform to give the same transfer function for the envelope while at the same time filtering any other noise on the signal so that determining its frequency is simplified.
The transfer function of the system is therefore:
      Δω    Δτ    =            1      +      sT                                s          2                ⁢        JT            +      sJ      +      k      
Which has a damping factor of:
  ζ  =            1      2        ⁢                  J        kT            
The system performance is therefore dependant on the system inertia, filtering constant and available controllable load. High gains and short time constants giving rapid response and high accuracy come at the expense of a low damping factor that is not acceptable. Thus the essence of control here is to always have enough inertia in the total power system. In our experience using a filter with a Q of 10 corresponding to a bandwidth of 5 Hz with control exercised over the range 49.5 to 50.5 Hz gives an acceptable response for inertias of greater than 0.02 kG.m2/2-pole kW. Thus for a 100 kW 6-pole machine the required inertia with a Q of 10 is 0.02×(6/2)2×100=18 kG.m2 which is a substantial inertia. Since inertia increases as the 4th power of the machine diameter times the length inertias are more readily achieved with larger machines. However the biggest problem with DDC is that the system frequency varies so it cannot be seamlessly integrated into a grid network.